The technique of creating two more stitches, or Double Increase, is an important aspect of knitting. This video is a collection of the best-known Double Increases, and includes the tutorials how to work them.
Written instructions are in the article on my website:
https://wordpress.com/page/assiabrill.com/1981
T-increase video: https://youtu.be/1q3p_TsA7Ec